Venezuela's economy is heavily dependent on oil. When oil prices rose, the standard of living in Venezuela had the tendency to rise with it. The second set of figures that you reference are during the first set of oil shocks where members of the oil cartel OPEC cut the US off from production. As a consequence oil prices rose dramatically. During the 80s, oil prices crashed and the Venezuelan economy suffered. The boom bust nature of oil has made economic development in Venezuela problematic. If you want to read a book that recently came out about this very subject, I would look into Ricardo Hausmann's Venezuela Before Chávez: Anatomy of an Economic Collapse.